Cyclization polyol

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

Oxidative processes (route B) represent another common route to triazolopyridines (compounds described in Schemes 54 and 55). These preparations all start from aldehyde hydrazones and use different oxidative reagents for the cyclization (Table 2). Generally, those conditions are milder than condensation methods. Moreover, the oxidizing reagents are compatible with other moieties, even the sugar-derived polyol 209. In the case of compound 208, the hydrazone (major diastereomer) was obtained by tautomerization of the corresponding enhydrazine, the... 

Dideoxyhex-2-enono-1,5-lactone derivatives (penten-5-olides) have been prepared (255-258) and employed as starting compounds in synthesis. Thus, Michael addition of benzylhydroxylamine to racemic 6-0-acetyl-2,3,4-trideoxy-D,L-g/ycerohex-2-enono-1,5-lactone (267) took place ster-eoselectively to give the unstable benzyloxyamino-2-pyrone 268, which was readily converted into the /Mactam derivative 269, a precursor of thienamy-cin (259). / -Lactams were also obtained (260) by 1,3-dipolar cycloaddition of nitrone 270 to the unsaturated 1,5-lactone 267, followed by hydrogenoly-sis and subsequent cyclization to the /Mactam 271, having a polyol side-chain at the C-3 position. 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

Hydrazone formation of pyridazine-3-hydrazines with aldoses, dialdofuranoses, and dialdopyranoses was studied by Stanovnik and co-workers. The respective hydrazones could be cyclized with Bt2 in MeOH or Pb(OAc)4 to j-triazolo[4,3-3]pyridazin-3-yl substituted polyols <1997JHC1115, 1998JHC513>. Similarly, 4-[(dimethylamino)-methylene]-l,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-one was reacted with pyridazine-3-hydrazines and the resulting mixtures were subsequently treated with Pb(OAc)4. Besides j-triazolo[4,3-3]pyridazine formation also diazenes were obtained. This can be rationalized by the enehydrazine-hydrazone mixtures observed in the first reaction. For phthalazin-l-hydrazines only diazenes were obtained after oxidation <2005TA2927>. Also cyclizations of... 

The chemistry presented in the previous two sections focussed on the formation of hydroxyolefins followed by cyclization to the desired C-glycosides. The latter steps in the examples shown rely largely upon the cyclization of protected polyols. In this section, cyclization methods including ether formations, ketal formations and halide displacements are presented. 

Among the simplest of the cyclizations utilized in the formation of C-glycosides is the acid-mediated formation of ethers. Such reactions can be viewed as dehydration between two alcohol units, with the driving force for the reaction being the elimination of water. As shown in Scheme 7.98, Schmidt and Frick [242] formed the perbenzylated polyol by treating the lithiated... 

Recently, the rapid preparation of carbohydrates has been facilitated by a synthetic route based on aldol coupling of three aldehydes used for the de novo production of polyol differentiated hexoses in only two chemical steps. The dimerization of alpha-oxyaldehydes, catalyzed by L-proline, is followed by a tandem Mukaiyama aldol addition-cyclization step catalyzed by a Lewis acid. Differentially protected glucose, allose, and mannose stereoisomers can each be selected, in high yield [46]. Microwave irradiation is becoming an increasingly popular method of carbohydrate synthesis and has been the subject of a recent review [47]. 

Johnson and Schaaf4 applied the method of CF3C02H-cyclization of polyole-finic allyl alcohols to the tetraenol (16) and obtained the two 13-epimeric alcohols (17) and (18) of the podocarpanc series. 

In 1992, the reported preparations of C-disaccharides included the utilization of free radical reactions. Additionally, the use of various polyol cyclizations were discussed. In the application of radical chemistry to the synthesis of C-disaccharides, Bimwala, et al.,35 expanded upon his earlier work illustrated in Scheme 8.9.5. As shown in Scheme 8.10.1, glycosidic radicals were added a,p-unsaturated carbonyl compounds producing yields ranging from 47% to 73%. Furthermore, the illustrated C-disaccharide precursors exhibited diastereomeric ratios of approximately 5.5 1. 

The cyclization of polyols to tetrahydropyran rings was discussed, in context with the formation of C-glycosides, in Chapter seven. Through... 

In Scheme 8.10.4, the above described protected polyol is converted to the illustrated hemiacetal via a Swern oxidation followed by spontaneous cyclization. The hemiacetal is converted to the final product through two... 

The strategy eluded to in Scheme 7.3.7 is elaborated upon in Scheme 8.10.5 and involved the starting material prepared in Scheme 8.10.3. Cyclization of the polyol is effected on removal of the acetonide under acidic conditions followed by the oxidative cleavage of the resulting diol. Spontaneous formation of the hemiacetal is followed by protection as the p-nitrobenzyl ether. Although two diastereomers are formed in this sequence, the minor product is easily converted to the major under mild basic hydrolysis of the p-nitrobenzyl ether followed by reprotection of the isomerized hemiacetal. This compound is then converted to the allene, shown, on treatment with propargyltrimethylsilane in the presence of trimethylsilyl triflate. 

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